The present invention relates to an improvement in an automatic transmission gear system for vehicles, and more particularly, it aims to reduce the shocks due to gear shifting and to economize fuel consumption.
Conventionally, an automatic transmission system employed in an automobile and the like operates by means of an oil pressure control device which controls a plurality of advance gears. In order to achieve automatic transmission, plural engagement devices of mechanical type such as a oneway clutch and of the type operated by oil pressure such as brakes and clutches are provided and are suitably selected and engaged.
For example, a oneway clutch is commonly used to alleviate the shocks caused when shifting a low speed gear, normally the first gear, to the second or vice versa. Although such oneway clutch is capable of transmitting the driving force of the engine to the wheels, it races and fails to function when it is necessary to apply engine brake. Another friction engagement device operated by oil pressure such as a brake is, therefore, provided in parallel with the oneway clutch so that as the brake is engaged by the supply of high pressure oil, the engine brake can function.
However, the engine brake at the lst speed is necessary only when the manual gear selection is at the L-range, and for the 2nd speed driving, engine brake at 2nd gear achieves the effect. In the D-range driving, the 1st speed gear is necessary only at the time of starting the vehicle.
It would therefore increase loss in the operation of the oil pump which supplies the brake with high pressure oil to engage the same for applying the engine brake at the 1st speed during driving in the ranges other than the L-range. Moreover, it is difficult to timely engage the friction engagement devices and release the brake for shifting the 1st gear to the second, and the difficulty often causes undesirable shocks in gear shifting. On the other hand, if the system is so constructed as to supply high pressure oil to the brake only during the L-range driving, this would then bring about such disadvantages as that the control circuit for the oil pressure becomes extremely complicated.